LLC converters have a topology that utilizes a combination of two inductors and one capacitor (“LLC”) on the primary side of the converter. A switching (full or half) bridge on the primary side generates a square waveform that excites the LLC tank circuit, which in response outputs a resonant sinusoidal current that is scaled and rectified by a transformer and rectifier circuit of the LLC converter. An output capacitor filters the rectified ac current and outputs a DC voltage.
Synchronous rectifier (SR) switches on the secondary side of an LLC converter are conventionally off during light-load conditions in which the output voltage is unregulated, and thus function as diode rectifiers. The PWM (pulse width modulation) pattern generation is simpler with this approach, avoids reverse power flow and reduces the likelihood of hard-commutation, but reduces the achievable efficiency due to higher losses in the built-in FET (field-effect-transistor) diodes.
A common driving scheme for the bridge switch devices on the primary side of an LLC converter uses a bootstrap capacitor. However, the bootstrap capacitor must be first charged in order to apply the proper PWM pattern. This is applicable for the converter start-up, but also in case of burst mode operation during light-load conditions since the off time can be long enough to discharge the bootstrap capacitor.
Typically, a large pulse of several switching cycles is applied to the low-side bridge switch device to charge the bootstrap capacitor to a sufficient level that allows proper PWM operation of the high-side bridge switch device later on. However, in the case of a split-capacitor design, the presence of clamping diodes in parallel with the series-connected split-capacitors causes unbalance in the split capacitor voltage. Particularly, the low-side capacitor gets fully discharged.
As a result of the unbalance in the resonant capacitor voltages, the resonant current does not change polarity before the PWM changes. Therefore, hard commutation occurs which can destroy the bridge switch devices on the primary side. Furthermore, there is an increased voltage stress in the secondary side SR switch devices which can damage the SR switch devices.
One conventional technique involves the use of asymmetrical PWM, with duty cycle different than 50%. In this case, the first PWM pulse presents a reduced duty cycle to ensure the resonant current polarity change. The same pattern can be applied in to resume operation during burst mode. If clamping diodes are present, the unbalance in the resonant capacitor voltages may require more than a single PWM pulse with duty cycle different than 50%, thus complicating the controller sequence.
A more complex conventional technique involves the use of the resonant current information to apply a time delay between the zero crossing of this current and the PWM change. This sequence completely avoids the hard commutation of the bridge switch devices on the primary side. However, resonant current sensing is needed for proper behavior. Furthermore, the timing is essential to avoid current stress in the circuit.
One disadvantage of the conventional techniques described above is that the SR switch devices on the secondary side of the LLC converter are turned off under certain load conditions. This leads to lower efficiency even in light-load operation, which can be further reduced depending on the switching frequency applied during burst mode operation. In burst mode, the LLC converter is turned off and then started again. The output voltage of the LLC converter increases in burst mode until the converter turns off again, at which point the output voltage begins to drop. Conventionally, all switch devices on the primary and secondary sides of the LLC converter are turned off (i.e. not switching) when the LLC converter is off. The secondary side switches are already off because of low current. When the output voltage increases to a certain level, the primary side switch devices are also turned off. When the output voltage drops to a certain level (under light-load conditions), the primary side switch devices are turned on again in burst mode while the secondary side switch devices remain off due to light-load conditions. Also problematic with the conventional techniques described above is that the use of the SR switch devices on the secondary side as diodes changes the gain behavior of the LLC converter. This gain change may lead to more burst mode operation, which increases the risk of hard commutation and additional stress in the switch devices.